How to Draw a Molecular Orbital Diagram: N2, O2, CO, F2 and H2 Examples

Molecular orbital diagrams are easy to recognize and surprisingly easy to get wrong. And the mistakes are rarely artistic ones. What usually goes wrong is the chemistry underneath: the orbital order gets swapped, the valence electron count is off, the two degenerate pi orbitals are filled the wrong way, or the bond order at the end simply does not match what the molecule actually does.

This guide walks through a practical way to draw MO diagrams for the common diatomic molecules, and then shows how to turn that into a clean teaching or publication figure using the SciDraw AI Molecular Orbital Diagram Generator.

A well-labeled N2 diagram shows the 2s and 2p atomic orbitals, the bonding and antibonding molecular orbitals, and all 10 valence electrons.

The Short Answer

Drawing a molecular orbital diagram comes down to six steps:

1. Count the valence electrons contributed by both atoms.
2. Draw the atomic orbitals on the left and the right.
3. Place the molecular orbitals in the correct energy order in the middle.
4. Fill electrons from low to high energy, respecting the Pauli exclusion principle and Hund's rule.
5. Calculate the bond order: (bonding electrons - antibonding electrons) / 2.
6. Check that the result actually explains the molecule's bond strength and magnetic behavior.

For second-row homonuclear diatomics, the one decision that trips people up is which ordering to use: the B2/C2/N2 ordering or the O2/F2 ordering. In B2, C2 and N2, s-p mixing pushes the pi 2p orbitals below sigma 2p. In O2 and F2, sigma 2p drops below the pi 2p orbitals.

The Mistakes Worth Avoiding

Mistake 1: Reusing one orbital order for every molecule

N2 and O2 do not share the same 2p ordering, and assuming they do is exactly what produces so many wrong O2 diagrams.

• For B2, C2, N2: pi 2p sits below sigma 2p.
• For O2, F2, Ne2: sigma 2p sits below pi 2p.

Mistake 2: Forgetting Hund's rule

When two molecular orbitals are at the same energy, like the pair of pi orbitals, put one electron in each before pairing any of them. This is precisely why O2 ends up with two unpaired electrons, and why it is paramagnetic.

Mistake 3: Drawing a pretty diagram and skipping the bond order

A diagram is not finished until you have checked the bond order. A visually clean MO diagram with the wrong bond order is still chemically wrong.

Mistake 4: Treating heteronuclear molecules like homonuclear ones

CO is not just "N2 with different atom labels." Oxygen's atomic orbitals lie lower in energy than carbon's, so the two atomic-orbital columns should be offset rather than drawn at the same height.

The Workflow, Step by Step

Step 1: Count the valence electrons

For a standard introductory MO diagram, count valence electrons only.

Molecule	Valence electrons	Expected bond order	Magnetic behavior
H2	2	1	Diamagnetic
N2	10	3	Diamagnetic
O2	12	2	Paramagnetic
F2	14	1	Diamagnetic
CO	10	3	Diamagnetic

Step 2: Choose the right orbital order

For N2:

sigma 2s
sigma* 2s
pi 2p
sigma 2p
pi* 2p
sigma* 2p

For O2 and F2:

sigma 2s
sigma* 2s
sigma 2p
pi 2p
pi* 2p
sigma* 2p

Step 3: Fill the electrons correctly

Fill from the bottom up. Show paired electrons with opposite arrows, and remember to fill degenerate pi orbitals singly before you start pairing.

Step 4: Calculate the bond order

Bond order = (bonding electrons - antibonding electrons) / 2

If the bond order doesn't match the chemistry you expect, go back and recheck the electron count, the orbital order, and how you filled the degenerate orbitals.

Example 1: The N2 Diagram

Nitrogen contributes five valence electrons per atom, so N2 has 10 in total. Since N2 falls in the B2/C2/N2 part of the second-row series, use the s-p mixing order, with pi 2p below sigma 2p.

The bond order works out to 3:

Bonding electrons: 8
Antibonding electrons: 2
Bond order = (8 - 2) / 2 = 3

A prompt that works well in SciDraw AI:

Create a molecular orbital diagram for N2. Show nitrogen atomic orbitals on the left and right, molecular orbitals in the center, the pi 2p orbitals below sigma 2p because of s-p mixing, 10 valence electrons filled as arrows, and a clear bond order label of 3.

Example 2: The O2 Diagram

Oxygen contributes six valence electrons per atom, giving O2 a total of 12. Here sigma 2p sits below the pi 2p orbitals, and the two highest electrons enter the degenerate pi* antibonding orbitals separately.

The O2 diagram should show two unpaired electrons sitting in the pi antibonding orbitals.*

That gives:

Bonding electrons: 8
Antibonding electrons: 4
Bond order = (8 - 4) / 2 = 2

Those two unpaired electrons are exactly why O2 is paramagnetic.

Example 3: The CO Diagram

CO has 10 valence electrons, just like N2, but it is heteronuclear. Because oxygen is the more electronegative atom, its atomic orbitals should be drawn lower than carbon's. A good CO diagram conveys polarity as much as bond order.

For CO, offset the carbon and oxygen atomic orbitals rather than drawing two identical side columns.

A SciDraw AI prompt:

Create a heteronuclear molecular orbital diagram for CO. Put carbon atomic orbitals on the left and oxygen atomic orbitals on the right, with oxygen orbitals lower in energy. Show asymmetric molecular orbitals, 10 valence electrons, lone-pair character, and a bond order of 3.

Example 4: The F2 Diagram

F2 has 14 valence electrons and uses the O2/F2 ordering. The extra electrons pile into antibonding orbitals, dropping the bond order to 1.

Bonding electrons: 8
Antibonding electrons: 6
Bond order = (8 - 6) / 2 = 1

That is why F2 has a single bond rather than a triple bond like N2.

Example 5: The H2 Diagram

H2 is the cleanest beginner example there is. Two hydrogen 1s orbitals combine into one sigma 1s bonding orbital and one sigma* 1s antibonding orbital, and the two electrons settle into the bonding orbital.

Bond order = (2 - 0) / 2 = 1

Reach for H2 when you want to teach the basic idea of constructive and destructive orbital combination before moving on to 2s and 2p diagrams.

Making the Diagram Publication-Ready

For a quick class note, a hand-drawn sketch may be all you need. For a paper, a lecture slide, or a textbook-style figure, the final graphic should have:

• consistent vertical spacing between energy levels,
• clearly labeled atomic and molecular orbitals,
• electron arrows large enough to read,
• bonding and antibonding labels,
• bond order and magnetic behavior annotations,
• a vector export, if you plan to edit the diagram in PowerPoint or Illustrator.

SciDraw AI can turn a chemistry prompt into a clean first draft, which you can then export or refine for teaching material, lab reports, and scientific presentations.

FAQ

What is the difference between an MO diagram and a Lewis structure?

A Lewis structure shows atom connectivity and electron pairs in a simplified 2D form. A molecular orbital diagram shows how the atomic orbitals combine into bonding and antibonding orbitals across the whole molecule.

Why is O2 paramagnetic in the MO diagram?

The O2 diagram has two unpaired electrons in degenerate pi* antibonding orbitals, and it is those unpaired electrons that make oxygen paramagnetic.

Can AI draw a correct molecular orbital diagram?

It can produce a genuinely useful draft, as long as the prompt spells out the orbital order, electron count, molecule type, and labels. Just always verify the bond order and magnetic behavior before the figure goes into class material or a publication.

If you want a head start, the dedicated tool lives here: https://sci-draw.com/molecular-orbital-diagram-generator.